Another circuit board isolation routed, this part of the jigsaw is a TTL switch. It will allow the selection of one from two inputs. Its purpose is to insert the 1uS 'tickle pulse' in-between the pauses in the PWM signal controlling laser output power.
Incidentally, for those who might be wondering, it only takes me a few seconds to remove my focus lens assembly and fit the 3 phase router spindle thus converting the machine from a laser to a router (not forgetting to fit the covers to prevent the dust getting onto my mirrors) so making these (simple) isolation routed PCB's is really quick and easy.
Tweakie.